Delta-sigma (DS) converters have been around for many years, and are a dominant form of converters used for high precision conversion of analog-to-digital signals and vice versa. Converters such as DS converters provide high dynamic range and flexibility in converting low bandwidth input signals. Delta-sigma (or sigma-delta) modulation is a signal processing method. It can be used, for example, to encode analog signals into digital signals as found in an ADC.
In a conventional ADC, an analog signal is integrated, or sampled, with a sampling frequency and subsequently quantized in a multi-level quantizer into a digital signal. This process introduces quantization error noise. Rather than quantizing an input signal's value, the first step in a delta-sigma modulation is to encode a change in an input signal (i.e., its “delta”). The result is a stream of pulses, as opposed to a stream of numbers as is the case with pulse code modulation (PCM). The next step in delta-sigma modulation is to then improve the accuracy of the modulation by passing the digital output through a one-bit DAC and adding (hence, a “sigma”) the resulting analog signal to the input signal, thereby reducing the error introduced by the delta-modulation.
This technique has found increasing use in modern electronic components such as converters, frequency synthesizers, switched-mode power supplies and motor controllers, primarily because of its high precision.
Both ADCs and DACs can employ delta-sigma modulation. A delta-sigma ADC first encodes an analog signal using high-frequency delta-sigma modulation, and then applies a digital filter to form a higher-resolution but lower sample-frequency digital output. On the other hand, a delta-sigma DAC encodes a high-resolution digital input signal into a lower-resolution but higher sample-frequency signal that is mapped to voltages, and then smoothed with an analog filter. In both cases, the temporary use of a lower-resolution signal simplifies circuit design and improves efficiency.
A standard delta-sigma converter circuit according to the prior art is schematically depicted in FIG. 1. Delta-sigma encoding is used in applications where high bit precision but limited frequency response is desired. To achieve this, the converter may have several feedback loops (that is, several integrator and summer junctions) and may have a multi-bit DAC. A delta-sigma converter oversamples the input signal and shapes the noise spectrum so that the modulator appears to be a high-pass filter for the noise and a low-pass filter for the input signal.
Continuous time delta-sigma converters however, have shortcomings. High frequency edge signals limit the operating frequency before noise and errors become dominant. Delta-sigma systems are typically limited in oversampling ratios. Delta-sigma systems typically employ multiple order, highly complex configurations to produce high quality results. These high quality results are achieved by introducing complexity into the system and hence increasing costs as component counts rise. In addition, the high quality results are achieved at the expense of power and extended group delay. These converters also have input overload limitations related to the output levels of the DAC. A system of three or more orders is likely to result in instability.
The frequency of operation of these systems is generally limited by the response of the differentiating (subtraction) hardware, typically an amplifier module. To overcome this, alternatives and modifications have been adopted including using lower frequency clocks, multi-bit DACs within the feedback loop with higher order digital filters and higher order modulators to achieve improved precision and noise performance. It is not uncommon to have five orders in a system. However, such modifications can cause the system to compromise intermodulation distortion (IMD), linearity, power dissipation, and group delay. Higher order modulators may not be completely stable.
Embodiments of the present disclosure provide circuits for generating pulse-width modulated waveforms that are particularly suitable for, but not limited to, controlling the known class-D circuit. Furthermore, embodiments of the present disclosure aim to ameliorate some of the disadvantages of the prior art